1. Field of the Invention
This invention relates to scale inhibition in industrial and commercial processes and plants. More particularly, it relates to the inhibition of scale formation by electrochemical means intended primarily, but not exclusively, for use in Bayer plants designed for the production of alumina from bauxite.
2. Background Art
The Bayer process is a well-known method of obtaining alumina for aluminum production from bauxite, the principal ore. The Bayer process circuit involves a series of digestion and precipitation steps carried out in a number of vessels that are interconnected by pipes and operated by a series of pumps and valves. Many of the steps of the process involve highly alkaline conditions and elevated temperatures and pressures. A problem that persists in such processes is that, as the process is operated, scale (i.e. a solid deposit that is difficult to remove) tends to form at various points in the apparatus. The scale formed in the Bayer process is usually gibbsite or sodalite (alumino-silicate salts containing sodium carbonate and sodium sulfate in addition to alumina and silica). This build-up of scale reduces the efficiency of the operation and may result in plant shut-down. Periodic scale removal is generally carried out, but this can result in expense and operational delays. For example, it has been calculated that the cost of the production of alumina could be reduced by 5 to 10% if scale formation could be avoided.
In the past, no commercially effective way of avoiding scale formation has been developed and effort has been concentrated instead on methods of scale removal. For example, U.S. Pat. No. 4,731,259 which issued on Mar. 15, 1988 to David J. Lloyd discloses a process for de-scaling surfaces of Bayer process equipment by first cleaning the surfaces and then coating the surfaces with a suitable resin, such as epoxy resin, that is thermosetting upon being cross-linked. The coating is applied in two or more layers and the final layer is one that readily detaches from the base coating when subjected to a high pressure fluid blast. Thus, scale that has built up on such a surface may be removed by high pressure fluid cleaning.
Clearly, even such procedures require a definite cleaning step that may cause delays in processing and even plant shut-down. It would therefore be advantageous to prevent the build up of scale in the first place so that cleaning and de-scaling operations may be avoided entirely, or at least delayed considerably.